On Thu, Oct 18, 2012 at 5:29 AM, Jassi Brar jaswinder.singh@linaro.org wrote:

On 18 October 2012 12:15, Huang Shijie b32955@freescale.com wrote:

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于 2012年10月18日 14:18, Vinod Koul 写道:

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Why cant you do start (prepare clock etc) when you submit the descriptor to dmaengine. Can be done in tx_submit callback. Similarly remove the clock when dma transaction gets completed.

I ever thought this method too.

But it will become low efficient in the following case:

Assuming the gpmi-nand driver has to read out 1024 pages in one _SINGLE_ read operation. The gpmi-nand will submit the descriptor to dmaengine per page. So with your method, the system will repeat the enable/disable dma clock 1024 time. At every enable/disable dma clock, the system has to enable the clock chain and it's parents ...

But with this patch, we only need to enable/disable dma clock one time, just at we select the nand chip.

If the clock is the dmac's property (not channels'), the toggling seems too aggressive. You could try using runtime_suspend/resume for clock disabling/enabling. How about employing autosuspend with a few ms delay?

Yes, employing the autosuspend is workable method too.o But it's a little more complicated then this patch. You have to create a thread or workqueue to do the job.

What's more, I think other DMA engine may also meets the same issue as the mxs-dma, such as the imx-sdma. It's somehow a common issue to disable the clocks when no one use the DMA engine. Do you also suggest employing the autosuspend? Why not use a simple way to handle this issue? When the dma device want to use a DMA engine, it just needs to issue a DMA_START command.

thanks Huang Shijie Huang Shijie